Studies of Angiotensin-II Conformations by Circular Dichroism.

Angiotensinamide II has been studied by circular-dichroism spectroscopy in comparison with some of its peptidic fragments and structural analogs. The following observations have been made. The stepwise examination of elongated peptides shows the appearance of a structural organisation at the heptapeptide stage. Evidence that C-terminal phenylalanine residue plays an important role on the overall conformation of the octapeptide is obtained. As already suggested for some other short peptides, circular-dichroism spectra of angiotensinamide II show dramatic perturbations of the peptidic region due to side-chain aromatic-chromophore overlapping. The contribution of prolyl residue in position 7 of the peptide appears to be critical; equilibrium between cis and trans forms is believed to be shifted toward one or the other conformer upon binding to receptor site. This would result in the adequate positioning of the phenylalanine residue, the biological importance of which has already been shown. [ABSTRACT FROM AUTHOR]